The field of the invention is that of helmet sights and displays. These optical devices are used essentially on aircraft. They comprise an image source and an optical assembly ensuring the collimation and the superposition of the said images on the exterior landscape, the assembly being mounted on the pilot's helmet. The images presented are essentially synthetic images intended for aiding piloting or the designating of objectives. These devices form part of a system which also comprises a device for detecting the posture of the helmet making it possible to servocontrol, for example, the weapons system or the image presented in the helmet display. Initial setup of the image of the helmet sight with respect to the aircraft is carried out by a reference alignment or capture of the posture detection device. This operation is carried out by sighting a reference test pattern in a collimator or a head-up sight previously harmonized with the aircraft's weaponry and navigation system.
The latest-generation helmet sights use the helmet's protection visor as projection and collimation mirror. FIG. 1 represents a simplified view of a helmet sight of this type. An image source represented by the rectangle 3 in this figure generates an image which is collimated towards the pilot's eye by the optics 2 and a part of the visor 1 specifically treated for this purpose. Most helmet sights possess only one visor for reasons of head-borne mass and mechanical simplification. This single visor is in general “missionable”, that is to say there exist various tints thereof as a function of the luminous conditions encountered in the course of the mission. In this case, the pilot chooses his visor at the start of the mission and retains it throughout the flight.
This single visor solution is not satisfactory from the operational point of view since it does not allow the use of the helmet sight under all the conditions of the mission in particular when the weather changes or when the flight is performed at the start or at the end of the day.
Helmet sights with two visors correspond better to round-the-clock 24 h/24 mission conditions. The main visor used for image projection is the clear visor for protection against bird strikes or dust and the sun visor is a second visor usable at the pilot's discretion. The visibility of the information presented in the helmet display in fine weather requires an image of high brightness. A sun visor whose uniform transmission is around 15% is used to increase the contrast. The sun visor is placed outside the projection visor so as not to decrease the brightness of the image of the helmet sight and to improve the contrast.
The optical system of a helmet sight such as represented in FIG. 1 is an off-axis system, that is to say the visor 1 is inclined in such a way that the collimation optics 2 and the image source 3 lie outside the visual field of the eye of the user 4 wearing the helmet 5. The visor 1 is in general of spherical form or close to a sphere. This arrangement gives rise to a slight deviation of sighting which increases with the curvature and the thickness of the visor and depends on the refractive index of the material of the visor. FIG. 2 represents an enlargement of the centre of the visor. As seen in this figure, the double refraction of an incident ray 13 corresponding to the line of sight on the two spherical faces 11 and 12 of the visor 1 occurs at slightly different angles of incidence, having regard to the large inclination θ of the visor 1, causing the shift α of the exterior line of sight. In this figure, the chain-dotted lines represent the normals to the diopters 11 and 12. This deviation is automatically compensated during the image alignment and posture detection during the manufacture of the helmet sight.
The sun visor is in general parallel to the clear visor and exhibits the same type of prismatic deviation. Lowering it therefore gives rise to a shift of the exterior image with respect to the image of the helmet sight, of the order of 5 mrad, that must be compensated through a new alignment with the reference collimator. This operation carried out in flight is tricky and often carried out in a rather imprecise manner because of the movements and vibrations of the aircraft. The prismatic deviation of the sun visor can be cancelled or greatly reduced by making a visor of variable thickness whose external and internal surfaces are determined so as to exhibit surfaces locally parallel to the lines of sight leaving the eye. Such a visor can be made but it exhibits the drawbacks of an additional mass of about 30 grams and of a transmission that is likewise variable with the thickness of the visor therefore in the field, the transmission being about 8% at the bottom of the visor and 15% at the top.